1. Field of the Invention
The present invention relates, in general, to a pixel circuit of a type employed in a display system using a current driven organic or other light-emission device as a light source.
2. Description of the Prior Art
Display systems commonly comprise an array of pixel circuits having an organic light-emitting device (OLED) as a light source and a driving circuit for driving the OLED in accordance with a received data signal. The OLED consists of a light-emitting polymer (LEP) layer sandwiched between an anode layer and a cathode layer. Electrically, the OLED operates as a diode whilst optically, the OLED emits light when forward biased with the brightness of the emitted light increasing as the forward bias current increases. By integrating the driving circuits of individual pixel circuits in the array using low-temperature polysilicon Thin Film Transistor (TFT) technology, it is possible to control the brightness of each individual OLED in order to provide a still or a moving image on the display.
Since an OLED is a current driven device, if the pixel circuit receives a voltage signal, a driver transistor or the like is required to supply an appropriate level of current to the OLED in response to the received voltage signal. An example of a known voltage driven pixel circuit for an active matrix OLED display is illustrated in FIG. 1. Referring to FIG. 1, a pixel circuit 10 comprises a first p-channel TFT T1 and a second p-channel TFT T2 per pixel. The first TFT T1 is a switch for addressing the pixel circuit 10 and comprises a terminal coupled to a first supply line 12 for receiving a voltage data signal VData. The first TFT T1 also comprises a gate terminal coupled to a second supply line 14 for receiving a supply voltage VSEL, and a terminal coupled to a gate terminal of the second TFT T2. The second TFT T2 comprises a terminal coupled to a third supply line 16 for receiving a supply voltage VDD, and a terminal coupled to an anode terminal of an OLED 18, a cathode terminal of the OLED 18 being coupled to ground. The second TFT T2 is an analogue driver TFT for converting the voltage data signal VData into a current signal that in turn drives the OLED 18 at a designated brightness.
Display systems employing an array of voltage driven pixel circuits as illustrated in FIG. 1 can experience non-uniformity problems in their displayed images even though individual driving TFTs in the array are supplied with an identical voltage data signal and supply voltage. The non-uniformity arises due to a spatial variation in the threshold voltage of individual driving TFTs within the array of pixel circuits that form the display. Each OLED is therefore driven at a different brightness corresponding to the difference in threshold voltage between the driving TFTs. One approach to solving the non-uniformity problem has been disclosed by S. M. Choi, et al. in “A self-compensated voltage programming pixel structure for active-matrix organic light emitting diodes”, International Display Workshop 2003, p535-538. A pixel circuit embodiment as disclosed by Choi et al., is illustrated in FIG. 2.
Referring to FIG. 2, a pixel circuit 20 for compensating voltage threshold variations of individual driving TFTs comprises six TFTs M1, M2, M3, M4, M5 and M6, one capacitor C1 and two horizontal control lines, scan[n−1] and scan[n]. M2, M3, M4, M5 and M6 are switching TFTs, and M1 is an analogue driver TFT for providing a current that in turn drives an OLED 22 at a designated brightness during a time period of one frame.
In operation, the fourth TFT M4 provides a current path to establish a gate terminal voltage of the driver TFT M1 at a predetermined value. The capacitor C1 is a storage capacitor and stores the gate terminal voltage of the driver TFT M1. Since the pixel circuit 20 requires two row line time to complete data programming operation, the scan[n] (present row scan) and the scan[n−1] (previous row scan) signals are applied to program the pixel circuit 20.
During the previous row scan, when the scan[n−1] signal is logic low, a gate terminal voltage of the driver TFT M1 is charged to a voltage VI in a step referred to as initialisation. Next and during the present row scan, when the scan[n] signal is logic low, TFT M2 and TFT M3 are turned on so that the voltage data signal data[m] is programmed to a gate node of the driver TFT M1 through diode connected driver TFT M1. At this time, the programmed voltage at the gate node of the driver TFT M1 is automatically reduced to a value data signal voltage data[m] less a threshold voltage VTH of the driver TFT M1. During initialisation and programming TFTs M5 and M6 are turned off.
Following the previous and present row scans, TFT M5 and TFT M6 are turned on by an em[n] signal to establish a current path from VDD to ground so that current can flow through the driver TFT M1 and drive the OLED 22. The driver TFT M1 therefore moderates the current independently of the voltage threshold VTH.
Although the above pixel circuit 20 provides a means for compensating voltage threshold variations of individual driving TFTs, there is a need to increase the speed at which a pixel circuit can be programmed because an increase in programming speed is necessary in order that display systems can perform adequately when supplied with high bandwidth data or when employed in large size displays. Furthermore, there is a need for smaller display systems featuring lower power consumption in order to prolong the life of the power supply and expand the functionality of the system.